This invention relates to high-speed precision positioning apparatuses useful for processing of devices such as semiconducting wafers, for micro-scale experimentation, or for high resolution electron microscopy of biological or other samples.
Semiconductor processing equipment often requires high-precision positioning of a silicon wafer. Large-size wafers with very small features often require high-precision positioning combined with a large range motion of the wafer stage that supports the wafer.
In addition to high precision and a large range motion, high-speed scanning is useful for achieving high manufacturing throughput.
A typical example of semiconductor processing equipment is a laser used in microlithography. Certain traditional systems for semiconductor laser processing include a translation stage that can support the wafer and move it under a fixed laser beam. The wafer-supporting translation stage may move along the X or Y directions, for example.
In certain processing equipment a linear motor and a bearing system are used to move the wafer-supporting stage along a rail in the X-direction. To move the wafer-supporting stage in the Y-direction an intermediate rail is used that carries the rail and the wafer-supporting stage in the Y-direction while allowing the wafer-supporting stage to slide along the rail in the X-direction.
In addition to drive motors, various moving parts, bearings, and wafer, the wafer-supporting stage may carry a position detector. The position detector may be a laser-distance interferometer that is usually heavy, and requires environmental shielding of the optical path and slow motion in order to achieve high accuracy readings.
Hollis, U.S. Pat. No. 5,153,494, describes an apparatus capable of moving light loads in X, Y, and yaw (rotation around the Z axis) in limited range and with high speed. Vibrations of the stationary portions of the apparatus are reduced by a momentum-canceling design. The momentum canceling design includes, in addition to the movable wafer-supporting stage, a non-coplanar moving element that moves in a momentum canceling manner with respect to the movable wafer-supporting stage. The motors include fixed permanent magnets and coils that are attached to the wafer-supporting stage and move along with the wafer-supporting stage.
Trumper, U.S. Pat. No. 5,196,745, describes an apparatus capable of providing movement in the 200 to 300 mm range in one or two degrees of freedom and having precision control in the 10 nm range in the remaining degrees of freedom. Linear motion of the wafer-supporting stage is provided by a motor having an array of permanent magnets attached to the movable wafer-supporting stage and a fixed array of commutating coils attached to the stator. The commutating coils have coupled magnetic flux lines. Current is allowed to flow only in those sections of the array where the motion takes place, in order to reduce energy losses in the coils. The motion is facilitated by magnetic bearings.
In one aspect, the invention features a device positioning apparatus having a stage configured to support the device, a platen supporting the stage, and a plurality of drive motors, co-planar with the stage and arranged around the stage, that apply drive forces directly to the stage without any mechanical contact to the stage. The drive forces impart planar motion to the stage in at least three degrees of freedom of motion. A plurality of fluid bearings operate between the stage and the platen and constrain the planar motion of the stage in all remaining degrees of freedom. A controller connected to the drive motors controls operation of the drive motors. Each drive motor has a top stationary coil, a bottom stationary coil, and a movable magnet adapted to move in a gap of predetermined length formed in-between the top and bottom stationary coils.
The arrangement according to the invention makes it possible, in certain embodiments, for there to be only one monolithic movable part. This arrangement allows the stage to be made very stiff, which in turn promotes use of very high servo gains in the controller. Benefits of this arrangement can include good precision, fast settling time, and small servo-tracking error.
Because the motors are provided in a movable magnet configuration, the drive coils can remain stationary, thus eliminating any need to attach leads to the movable stage. This arrangement improves the reliability of the coils, because their leads are not flexed.
In some embodiments the stage may be a small-motion stage, and the apparatus may include a large-motion stage that moves on a plane parallel to a plane of motion of the small-motion stage. This arrangement allows a heavy load processing equipment, e.g., laser, to be carried by a low performance large-motion element that can scan the full range of the wafer. The light load wafer can be supported by a high-precision, high-speed, small-motion stage that scans the small range of the individual devices on the wafer.
In another aspect, the invention includes a force cancellation system that applies cancellation forces to the device positioning apparatus. The cancellation forces are co-planar with the center of gravity of the stage and any components that move with the stage and cancel forces generated by the planar motion of the stage. Because both the drive forces and cancellation forces are on the same plane, no net moments are created out of the plane that may cause deflections in the stage and may result in lower precision. This planar configuration results in high servo loop stiffness, a fast settling time, and high precision.
In another aspect, the invention features a position detector that includes at least one interferometric encoder. The encoders are insensitive to environmental disturbances such as change in temperature, humidity, or barometric pressure, thus reducing the need for controlling the ambient environment of the positioning apparatus and thereby reducing cost. Interferometric encoders can be relatively inexpensive, have low weight, and can be configured in a moving grid configuration, which keeps the read head electronics and associated leads stationary. Reliability can be improved by eliminating lead flexing with movement of the stage.
In another aspect, the invention features a controller that controls the operation of the drive motor and includes a trajectory planner that receives a description of a desired trajectory, produces stage drive commands and force cancellation commands for the drive motors and the force cancellation system, respectively, and compensates for phase lag between the stage drive commands and the force cancellation commands. The controller also includes a digital controller that receives commands from the trajectory planner and feedback signals, and compares the feedback signals to the commands and generates correction signals for the drive motors.